Attention Deficit Hyperactivity Disorder (ADHD) is a neurobehavioral disorder that impairs the normal development of behavior, in particular of executive functioning. Three persistent symptoms characterize ADHD: inattention, impulsivity, and overactivity. A large body of evidence suggests that the Spontaneously Hypertensive rat (SHR) displays all three symptoms of ADHD, and may thus serve as a model for experimental research. The validity of SHR as a model of ADHD has, nonetheless, been challenged. A concern with animal models is that they never perfectly replicate the syndrome-they are at best models of symptoms of that syndrome. This application focuses on one symptom of ADHD, persistent overactivity. The primary aim of this research is to characterize SHR overactivity, grounded on state-of-the-art behavioral theory; its results will serve as foundation for the evaluation of this strain as a model of ADHD-related overactivity. In particular, this application s concerned with instrumental overactivity, the excessive emission of rewarded responses that is typical of the predominantly hyperactive-impulsive and combined subtypes of ADHD. The proposed methods are supported by empirically-validated models of instrumental behavior that indicate that rats engage in rewarded activities in bouts separated by pauses. Response rate may be thereby partitioned by 3 parameters: within-bout response rate, bout-initiation rate, and bout duration. Past research has shown the differential sensitivity of each parameter to motor, motivational, and schedule manipulations: Each is controlled by a distinct mechanism linked with motor capacity, motivational status, and the maintenance of response-reward associations (coupling). Preliminary research suggests that young-adult SHR produce shorter but more frequent response bouts than its normoactive control, the Wistar-Kyoto (WKY) rat. Those results indicated that a reduced response-reward coupling was responsible for the differences between strains. This important finding is compromised by the age of the rats and the use of multiple interacting schedules. Exp. 1 will seek to replicate the results from the preliminary study, but exposing adolescent (instead of adult) SHR and WKY to separate schedules (instead of multiple schedules), in which rewards will be programmed every 12 or 192 s. Exp. 2 will validate the interpretation of strain differences by resynthesis: in this case, by experimentally reversing their main trait difference. For instance, if Exp. 1 verifies a response-reward coupling deficit in SHR on the basis of short-but-frequent bouts of responses, coupling would be increased and bouts normalized by rewarding longer response sequences. It will be verified whether such normalization generalizes to extinction performance. Taken together, these experiments will qualify our confidence in the preliminary findings (Exp. 1) and their interpretation (Exp. 2). Establishing the source of SHR overactivity is a fundamental step to its validation as a model of ADHD-related overactivity. The development on such model is critical for the progress of neurobiological research on ADHD and for the formulation of treatment alternatives.